Multiporosity electrode for electrochemical conversion

ABSTRACT

In an electrochemical process, the reaction takes place within the confines of a porous electrode element in which the pores of the lower portion have a lower effective size than the pores of the upper portion.

iiited States Patent 1 Ashe, Jr. et a1. l 1Marh 13, 1973 i 1MULTIPOROSITY ELECTRODE FOR [56] References Cited ELECTROCHEMICALCONVERSION UNITED STATES PATENTS [75] Inventors: Benedict H. Ashe, Jr.,Bartlesville;

3,113,048 12/1963 Thompson ..204/299 321" Austm both of 3,393,249 7/1968Fox et a1 3,385,780 5/1968 Feng ..204/294 [73] Assignee: PhillipsPetroleum Company, Bar- 3,423,247 1/1969 Darland, Jr. et al ..136/120tlesville,Okla. 3,335,034 8/1967 Laurent et al ..136/120 [22] Filed:July 1970 Primary Examiner-John H. Mack [21] Ap l, N 58,952 AssistantExaminer-Regan J. Fay

Att0rney-Young & Quigg Related U.S. Application Data [62] Division ofSer. No. 739,510, June 24, 1968, Pat. No. [57] ABSTRACT In anelectrochemical process, the reaction takes place within the confines ofa porous electrode ele- ..204/284l;62{):/32/3: mem in which the pores ofthe lower portion have a [58] 7 Field 294 59 lower effective size thanthe pores of the upper portion.

14 Claims, 5 Drawing Figures MULTIPOROSITY ELECTRODE FOR ELECTROCHEMICALCONVERSION BACKGROUND OF THE INVENTION This application is a division ofcopending application Ser. No. 739,510, filed June 24, 1968, now U.S.Pat. No. 3,558,450.

This invention relates to electrode elements and processes forelectrochemical conversions.

Porous electrode elements, particularly porous carbon anodes, are widelyused in electrochemical conversion reactions. Generally the utilizationof such elements has involved immersing the element in an electrolyteand passing an electric current through this electrolyte from thiselement to an oppositely charged element. At least a portion of thematerials within the electrolyte is converted into products at one orboth electrodes. In a variation on this process, an additional feedstockfor the conversion process is bubbled into the electrolyte through aporous electrode element, such as a porous carbon anode, to producestill different products.

Very recently it has been discovered that the reaction in anelectrochemical conversion operation can be carried out within theconfines of the porous electrode element itself. This type of operationis of particular utility in electrochemical fluorination because itmakes possible a simple one-step route to partially fluorinated productswhich had previously been difficult to obtain. Carrying out thefluorination reaction within the porous anode, in addition to makingpossible the direct production of partially fluorinated products, alsoallows operation at high rates of conversion, and without the formationof substantial amounts of cleavage products generally produced by theolder methods when operating at high conversion rates. The feed to befluorinated is introduced into the porous anode at a point near itsbottom and the fluorinated mixture exits at the top of the anode,generally above the electrolyte level. Passage of the feed into the bulkof the electrolyte is avoided.

It is apparent that if the reaction is to take place within theelectrode element, larger electrodes are desirable in order to increasethe available surface area wherein the reaction takes place. However,with larger electrodes, it has been found that an uneven distribution offeed material can occur within the electrode with the result forinstance in fluorination reactions using a KF-2HF electrolyte that inportions of the electrode farthest from the feed entry port, there isdeveloped an excess of fluorination species and a deficiency of feed tobe fluorinated. This produces unsatisfactory results among which is theproduction of high amounts of perfluoro materials which seriously reducethe advantage of this system for the production of partially fluorinatedproduct.

It would appear that by drilling feed distribution channels such aslateral passages through the bottom of the electrode element, evendistribution of the feed could be obtained. Thus the advantages ofcarrying out the reaction within the electrode, namely production ofmoderately fluorinated products and operation at high conversion rateswithout production of significant amounts of cleavage products, could beobtained in an electrode which was relatively wide and of sufficientsize to be employed in a commercial scale operation.

However, when attempts were made to increase the size of the electrodesstill more by making them longer for deeper immersion in theelectrolyte, it was found that electrode elements having such feeddistribution cavities tended, after a time, to undergo invasion withelectrolyte which interfered with the feed distribution and ultimatelyresulted in plugging with material which is solid at the operatingtemperature; in such instances the process either stopped or reverted tothe production of a large proportion of perfluoro compounds and/orcleavage products due to the resulting uneven distribution of feed. Thisproblem was particularly acute when the electrodes were submerged to adepth of more than about 6 inches in the electrolyte.

The invasion problem could not be solved by simply using carbons oflower porosity and smaller pore size because these tended to bubble feedinto the electrolyte at these greater immersion depths and at desirablefeed rates. Such tight carbons were also more subject to frequentpolarization (anode effect) than were loose carbons. Polarization is anincompletely understood phenomenon wherein the resistance of the cellsuddenly rises and the cell simply stops operating.

SUMMARY OF THE INVENTION It is an object of this invention to provide animproved process and apparatus wherein the reaction in anelectrochemical conversion process using a porous electrode element iscarried out within the confines of the electrode element; it is afurther object of this invention to prevent invasion of feeddistribution channels in an electrochemical conversion electrodeelement; it is yet a further object of this invention to provide foruniform distribution of feed through a porous electrode element duringlong periods of continuous operation; it is a still further object ofthis invention to provide even distribution of feed material in arelatively large electrode element suitable for commercial scale use.

which the reaction takes place within the confines of the porouselectrode element.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings, forming a parthereof, in which like reference characters denote like parts in thevarious views, FIG. 1 is a schematic representation of anelectrochemical cell arrangement utilizing an electrode having variableor multiporosity; FIG. 2 is a schematic representation of anotherelectrode element having multiporosity; FIG. 3 is a cross-sectional viewalong section lines 3-3 of FIG. 2; FIG. 4 is a view, in cross section,of a variable porosity cylindrical electrode in accordance with thisinvention; and FIG. 5 is a side view, partially in section, of anothercylindrical electrode in accordance with this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The smaller effective size ofthe pores in the lower portion of the porous electrode element can beachieved by utilizing a porous electrode element fabricated initiallywith smaller pores in the lower section or by selectively plugging orpartially filling the pores in the lower section, particularly thelarger ones.

A porous electrode element of carbon, for instance, can be fabricatedinitially with smaller pores in the lower section by forming an integralunit in a conventional manner from the pyrolysis product of petroleumpitch, the usual raw material for such porouscarbon, the differencebeing that two different particle sizes are utilized. The material isparticulated, graded and mixed with a suitable binder and charged to amold using con ventional technology except that small particle size mixis placed in one end of the mold and large particle size mix is put inthe other end. Alternatively two sections of anode material, one havinguniformly large pores and the other uniformly small pores can becemented together with a conventional porous cement to provide the dualporosity electrode element of the instant invention.

The specific pore size of the upper and lower sections will dependsomewhat upon the immersion depth in the electrolyte intended for theelectrode. That is, for use at relatively great immersion depths, thepores of the upper and lower section of the electrode should be somewhatsmaller than for a corresponding shorter electrode used at relativelyshallow depths. In all cases, however, the pore size of the lowersection which contains the feed introduction means in the form of feedentry ports and/or distribution passageways to be protected fromelectrolyte invasion, should be such that the electrolyte will notmigrate into said means at the specific operating depth being utilized.Similarly, the size of the pores in the upper section of the electrodeshould be such that will permit adequate movement of the fluid feedmaterial without using excessive pressures which might also cause thefluid feed to break out into the bulk of the electrolyte. Generallyspeaking, the fine pore section of the electrode should only be largeenough to enclose the ports and/or distribution passageways. Forexample, for a 12-inch immersion depth, a 14-inch electrode requires aline pore section of only about 1-% inches from the bottom, such sectioncontaining the feed introduction and distribution cavities. Generally,the fine pore section comprises the bottom l/50 to A, preferably l/25 toH7, of the total height of the electrode element.

The pore size in the fine pore lower section will generally be withinthe range of about 0.01 to 35, preferably 0.1 to microns averagediameter with no significant amount of pores having a diameter exceeding70 microns. The pore size of the upper portion of the electrode elementwill generally be within the range of about 40 to 150, preferably 50 to120 microns average pore diameter. The permeability of the lower portionhaving the fine pores will generally be within the range of about 0.001to 4, preferably 0.02 to 0.5, darcys. The permeability of the upperportion will generally be within the range of about 5 to 75, preferablyto 70, darcys. In general, the permeability should complement the poresize to carry out the desired function. That is, the permeability shouldbe high enough to prevent excessive pressure drop and the poredimensions should be such as to discourage undesirable electrolytepenetration.

The effective pore size can also be decreased in the lower portion of anelectrode of uniform pore size by impregnating the lower portion of saidelectrode with a polymer. Suitable polymers are normally solid polymersof at least one mono-l-olefin having two to eight carbon atoms permolecule, and fluorocarbon polymers and copolymers. Preferred polymersare homopolymers and copolymers of ethylene, propylene and butene, andhomopolymers and copolymers of tetrafluoroethylene andhexafluoropropylene.

The impregnation of the porous electrode element can be carried out byconventional means, such as by preparing a solution of the material tobe impregnated and dipping or otherwise contacting the porous electrodeelement with such a solution. The solution can be made using solventswhich will dissolve a suitable amount of the materials to beimpregnated. Convenient solvents are benzene, toluene, xylene,cyclohexane, and the like, and mixtures thereof. It is also within thescope of the present invention to impregnate the porous carbon with anaqueous or nonaqueous dispersion of the finely divided polymer.

The extent of the impregnation will be such that from about 0.0125 toabout 0.4, preferably from about 0.06 to about 0.2, gram of polymer willbe deposited in the porous electrode material for each cubic centimeterof void space. In general, it is preferred to treat only that portion ofthe electrode which contains cavities used for feed entry and feeddistribution. In most applications, cavities such as these are presentin the lower section of the electrode and, thus, only the lower sectionis treated.

After the porous carbon electrode has been impregnated with thepolymer-containing medium, the electrode is then dried at temperaturesbelow the decomposition temperature of the polymer to remove solventsand/or other volatile materials. Such drying can be carried out byconventional means such as by drying in an oven, preferably in thepresence of an inert gas.

The effective pore size can also be decreased by impregnating with anorganic material and subsequently carbonizing said organic material toleave a deposit of carbon.

The organic materials which are applicable for impregnation of theporous carbon are essentially any material which can be convenientlypyrolyzed leaving a carbon residue. In general, it is more efficient topyrolyze relatively nonvolatile materials such as polymers. Aparticularly preferred material is polymerized vinylidene chloride(commercially known as Saran). This material pyrolyzes cleanly atrelatively low temperatures by the loss of hydrogen chloride leavingcarbon behind. Other materials which can be used are poly(vinylidenefluoride), furfural polymer, cellulose nitrate, cellulose acetate, andthe like.

The impregnation of the porous carbon can be carried out by conventionalmeans, such as by preparing a solution or dispersion of the material tobe impregnated and dipping or otherwise contacting the porous carbonwith such a solution or dispersion. The solution can be made usingsolvents which will dissolve a suitable amount of the material to beimpregnated. Some convenient solvents are acetone, methyl ethyl ketone,

benzene, toluene, and the like, and mixtures thereof, depending upon thespecific materials to be dissolved.

The extent of the impregnation will be such that from about 0.0125 toabout 0.4, preferably from about 0.06 to about 0.2, gram of carbon willbe deposited in the porous electrode material for each cubic centimeterof void space.

The pyrolysis of the impregnated porous carbon material can be carriedout by subjecting the porous electrode element impregnated with saidorganic material to a sufficiently high temperature and for a sufficienttime to decompose the impregnant to carbon with the simultaneous removalof any gaseous byproducts. The pyrolysis is carried out in the absenceof air and generally in the presence of an inert gas. For example, whenpoly(vinylidene chloride) is used as the impregnant, the pyrolysisconditions can be 350 C. or higher for several hours in the presence ofa flowing inert gas such as nitrogen.

in embodiments where the pores in the lower section are treated, thiscan be done either before or after the formation of feed entry portsand/or distribution passageways.

The porous element of the electrode assemblies of the invention can befabricated from any suitable conducting porous electrolyte resistantmaterial which is compatible with the system, e.g., nickel, iron,various metal alloys, and carbon, which is not wetted by theelectrolyte. By not wetted is meant that the contact angle between theelectrolyte and the electrode must exceed 90 in order that anticapillaryforces will prevent substantial invasion of the small pores of theporous element by the electrolyte. Porous carbon, which is economicaland readily available in ordinary channels of commerce, is presentlypreferred for said porous element. In many instances it may beadvantageous to provide a metal element in contact with the porouselement. For instance, a porous carbon anode can have a nickel screenwrapped around it.

Various grades of porous carbon can be used in the practice of theinvention. It is preferred to employ porous carbon which has been madefrom carbon produced by pyrolysis, and not graphitic carbon. Suitableporous carbons for the embodiments of the invention employingimpregnation of the electrode elements are, broadly, those havingaverage pore diameters within the range of from 1 to 150 microns and apermeability within the range of 0.5 to 75 darcys. Carbons which willbenefit most from the invention will have an average pore diameter of40-140, preferably 50420, microns, and a permeability of 5-75,preferably -70, darcys. The electrodes of the invention can befabricated in any suitable shape or design, but must be arranged orprovided with a suitable means .for introducing the feed reactantmaterial into the pores of the porous element thereof.

The electrode assemblies of the invention can be employed in anyconvenient cell configuration or electrode arrangement. The onlyrequirements are that the cell body and the electrodes in the cell befabricated of materials which are resistant to the action of thecontents of the cell under the reaction conditions. Materials such assteel, iron, nickel, polytetrafluoroethylene (Teflon), carbon, and thelike, can be employed for the cell body. When a nonporous cathode or anonporous anode is employed which is not fabricated in accordance withthis invention (along with a porous anode or a porous cathode of theinvention), said nonporous cathode or nonporous anode can be fabricatedin any suitable shape or design and can be made of any suitableconducting material such as iron, steel, nickel, alloys of said metals,and carbon. For example, a nonporous cathode can be fabricated from ametal screen or gauze, a perforated plate, and can have a shapecomplementary to the shape of the porous anode.

The electrode assemblies of the invention can be employed in a widevariety of electrochemical conversion processes in which the porouselectrode is not wetted by the particular electrolyte being used andwherein the reaction takes place within the pores of the porouselectrode element. Some examples of such processes are electrochemicalhalogenation, electrochemical cyanation, and cathodic conversions suchas the reduction of alcohols to hydrocarbons or the reduction of acidsto alcohols. One electrochemical conversion process in which theelectrode assemblies of the invention are particularly valuable is theelectrochemical fluorination of fluorinatable materials in the presenceof an essentially anhydrous liquid hydrogen fluoridecontainingelectrolyte. Thus, for purposes of convenience, and not by way oflimitation, the electrode assemblies of the invention are primarilydescribed in terms of being employed as an anode in the electrochemicalfluorination of fluorinatable materials when using said hydrogenfluoride-containing electrolyte.

By preventing excessive invasion of the electrode element withconsequential flooding and blocking of the feed introduction area, theinstant invention allows operating with larger electrode elements whichcan be more deeply immersed in the electrolyte. With the passages withinthe electrode element suitably protected in this manner, the majorportion of the elements can comprise the relatively large pore materialwhich is more suitable as a reaction zone and which is less oftensubject to polarization.

As referred to hereinabove, the instant invention is of particularutility in the electrochemical fluorination wherein a current-conductingessentially anhydrous liquid hydrogen fluoride electrolyte iselectrolyzed in an electrolysis cell provided with a cathode and aporous anode (preferably porous carbon), a fluorinatable organiccompound is introduced into the pores of said anode and therein at leasta portion of said organic compound is at least partially fluorinatedwithin the pores of said anode, and fluorinated compound products arerecovered from said cell. The present invention provides improvedelectrode assemblies which are especially suited to be employed asanodes to produce partially fluorinated materials and/or to fluorinateorganic compounds with little or no scission of carbon to carbon bonds.

Very few organic compounds are resistant to fluorination. Consequently,a wide variety of feed materials, both normally liquid and normallygaseous compounds, can be used as feedstocks in this process. Organiccompounds which are normally gaseous or which can be introduced ingaseous state into the pores of a porous anode under the conditionsemployed in the electrolysis cell, and which are capable of reactingwith fluorine, are presently preferred as starting materials. However,starting materials which are introduced into the pores of the anode inliquid state can also be used. Generally speaking, desirable organicstarting materials which can be used are those containing from one toeight, preferably one to six, carbon atoms per molecule. However,reactants which contain more than eight carbon atoms can also be used.If desired, suitable feed materials having boiling points above celloperating temperatures can be passed into the pores of the porous anodein gaseous state by utilizing a suitable carrier gas. Thus, a suitablecarrier gas can be saturated with the feed reactant (as by bubbling saidcarrier gas through the liquid reactant), and then passing the saturatedcarrier gas into the pores of the porous anode. Suitable carrier gasesinclude the inert gases such as helium, argon, krypton, neon, xenon,nitrogen, etc. Normally gaseous materials such as hydrocarbonscontaining from one to four carbon atoms can also be used as carriergases. These latter gases will react, but in many instances this willnot be objectionable. The above-described carrier gases, andparticularly said inert gases, can also be used as diluents for the feedstocks which are normally gaseous at cell operating conditions.

Some general types of starting materials which can be used include,among others, the following: alkanes, alkenes, alkynes, amines, ethers,esters, mercaptans, nitriles, alcohols, aromatic compounds, andpartially halogenated compounds of both the aliphatic and aromaticseries. It will be understood that the abovenamed types of compounds canbe either straight chain, branched chain, or cyclic compounds. Partiallychlorinated and the partially fluorinated compounds are the preferredpartially halogenated compounds. The presently preferred startingmaterials are the saturated and unsaturated hydrocarbons (alkanes,alkenes, and alkynes) containing from one to six carbon atoms permolecule. The presently more preferred starting materials are thenormally gaseous organic compounds, and particularly said saturated andunsaturated hydrocarbons, containing from one to four carbon atoms permolecule.

Since fluorine is so reactive, no list of practical length could includeall starting materials which can be used. However, representativeexamples of the abovedescribed starting materials include, among others,the following: methane; ethane; propane; butane; isobutane; pentane;n-hexane; n-octane; cyclopropane; cyclopentane; cyclohexane;cyclooctane; 1,2- dichloroethane; l-fluoro-2-chloro-3-methylheptane;ethylene; propylene; cyclobutene; cyclohexene; 2- methylpentene-l;2,3-dimethylhexene-2; butadiene; vinyl chloride; 3-fluoropropylene;acetylene; methylacetylene; vinylacetylene; 4,4-dimethylpentyne- 2;allyl chloride; methylamine; ethylamine; diethylamine;2-amino-3-ethylpentane; 3- bromopropylamine; triethylamine; dimethylether; diethyl ether; methyl ethyl ether; methyl vinyl ether; 2-iodoethyl methyl ether; di-n-propyl ether; methyl formate; methylacetate; ethyl butyrate; ethyl formate; namyl acetate; methyl2-chloroacetate; methyl mercaptan; ethyl mercaptan; n-propyl mercaptan;2-mercaptohexane; 2-methyl-3-mercaptoheptane; acetonitrile;propionitrile; n-butyronitrile; acrylonitrile; n-hexanonitrile;methanol; ethanol; isopropanol; n-hexanol; 2,2-dimethylhexanol-3;n-butanol; ethylenebromohydrin; benzene; toluene; cumene; oxylene;p-xylene; and monochlorobenzene.

In addition to such fluorinatable organic materials described above,carbon monoxide and oxygen can be used as feedstocks to produce carbonylfluoride and oxygen difluoride, respectively.

The electrochemical fluorination process is carried out a medium ofhydrogen fluoride electrolyte. Although said hydrogen fluorideelectrolyte can contain small amounts of water, such as up to about 5weight per cent, it is preferred that said electrolyte be essentiallyanhydrous. The hydrogen fluoride electrolyte is consumed in the reactionand must either continuously or intermittently placed in the cell.

Pure anhydrous liquid hydrogen fluoride is nonconductive. Theessentially anhydrous liquid hydrogen fluorides described above have alow conductivity which, generally speaking, is lower than desired forpractical operation. To provide adequate conductivity in theelectrolyte, and to reduce the hydrogen fluoride vapor pressure at celloperating conditions, an inorganic additive can be incorporated in theelectrolyte. Examples of suitable additives are inorganic compoundswhich are soluble in liquid hydrogen fluoride and provide effectiveelectrolytic conductivity. The presently preferred additives are thealkali metal (sodium, potassium, lithium, rubidium, and cesium)fluorides and ammonium fluoride. Other additives which can be employedare sulfuric acid and phosphoric acid. Potassium fluoride, cesiumfluoride, and rubidium fluoride are the presently preferred additives.Potassium fluoride is the presently most preferred additive. Saidadditives can be utilized in any suitable molar ratio of additive tohydrogen fluoride within the range of from 1:4.5 to 1:], preferably 1:4to 1:2. The presently most preferred electrolytes are those whichcorrespond approximately to the formulas KF'ZHF, KF-3HF, or KF-4HF. Suchelectrolytes can be conveniently prepared by adding the requiredquantity of hydrogen fluoride to KF-HF (potassium bifluoride). Ingeneral, said additives are not consumed in the process and can be usedindefinitely. Said additives are frequently referred to as conductivityadditives for convenience.

The electrochemical fluorination can be effectively and convenientlycarried out over a broad range of temperatures and pressures limitedonly by the freezing point and the vapor pressure of the electrolyte.Generally speaking, the fluorination process can be carried out attemperatures within the range of from minus to 500 C. at which the vaporpressure of the electrolyte is not excessive, e.g., less than 250 mm Hg.It is preferred to operate at temperatures such that the vapor pressureof the electrolyte is less than about 50 mm Hg. As will be understood bythose skilled in the art, the vapor pressure of the electrolyte at agiven temperature will be dependent upon the composition of saidelectrolyte. It is well known that additives such as potassium fluoridecause the vapor pressure of liquid hydrogen fluoride to be dcreased anunusually great amount. A presently preferred range of temperature isfrom about 60 to about 105 C. Higher temperatures sometimes tend topromote fragmentation of the product molecules.

Pressures substantially above or below atmospheric can be employed ifdesired, depending upon the vapor pressure of the electrolyte asdiscussed above. In all instances, the cell pressure will be sufficientto maintain the electrolyte in liquid phase. Generally speaking, theprocess is conveniently carried out at substantially atmosphericpressure. It should be pointed out that a valuable feature of theprocess is that the operating conditions of temperature and pressurewithin the limitations discussed above are not critical and areessentially independent of the type of feed employed in the process.

For purposes of efficiency and economy, the rate of direct current flowthrough the cell is maintained at a rate which will give the highestpractical current densities for the electrodes employed. Generallyspeaking, the current density will be high enough so that anodes ofmoderate size can be employed, yet low enough so that the anode is notcorroded or disintegrated under the given current flow. Currentdensities within the range of from 30 to 1000, or more, preferably 50 to500, milliamps per square centimeter of anode geometric surface area canbe used. Current densities less than 30 milliamps per square centimeterof anode geometric surface area are not practical because the rate offluorination is too slow. The voltage which is employed will varydepending upon the particular cell configuration employed and thecurrent density employed. In all cases, under normaloperatingconditions, however, the cell voltage or potential will be lessthan that required to evolve or generate free or elemental fluorine.Voltages in the range of from 4 to 12 volts are typical. The maximumvoltage will not exceed 20 volts per unit cell. Thus, as a guide,voltages in the range of 4 to 20 volts per unit cell can be used.

As used herein unless otherwise specified, the term anode geometricsurface refers to the outer geometric surface area of the porous carbonelement of the anode which is exposed to electrolyte and does not include the pore surfaces of said porous element.

The feed rate of the fluorinatable material being introduced into thepores of the porous carbon element of the anode is an important processvariable in that, for a given current flow or current density, the feedrate controls the degree of conversion. Similarly, for a given feedrate, the amount of current flow or current density can be employed tocontrol the degree of conversion. Gaseous feed rates which can beemployed will preferably be in the range of from 0.5 to milliliters perminute per square centimeter of anode geometric surface area. With thehigher feed rates, higher current density and current rates areemployed. Since the anode can have a wide variety of geometrical shapes,which will affect the geometrical surface area, a sometimes more usefulway of expressing the feed rate is in terms of anode cross-sectionalarea (taken perpendicular to the direction of flow). On this basis, theflow rates can be 3-600, preferably 25 to 500, milliliters per minuteper square centimeter of cross-sectional area.

The actual feed rate employed will depend upon the type of carbon usedin fabricating the porous element of the anode as well as several otherfactors including the nature of the feedstock, the conversion desired,current density, etc., because all these factors are interrelated and achange in one will affect the others. The feed rate will be such thatthe feedstock is passed into the pores of the anode, and into contactwith the fluorinating species therein, at a flow rate such that theinlet pressure of said feedstock into said pores is essentially lessthan the sum of (a) the hydrostatic pressure of the electrolyte at thelevel of entry of the feedstock into said pores and (b) the exitpressure of any unreacted feedstock and fluorinated products from saidpores into the electrolyte. Said exit pressure is defined as thepressure required to form a bubble on the outer surface of the anode andbreak said bubble away from said surface. Said exit pressure isindependent of hydrostatic pressure. Under these flow rate conditionsthere is established a pressure balance between the feedstock enteringthe pores of the anode from one direction and electrolyle attempting toenter the pores from another and opposing direction. This pressurebalance provides an important feature in that essentially none of thefeed leaves the anode to form bubbles which escape into the main body ofthe electrolyte. Essentially all of the feedstock and/or reactionproduct travels within the carbon anode via the pores therein until itreaches a collection zone within the anode from which it is removed viaa conduit, or until it exits from the anode at a point above the surfaceof the electrolyte.

The more permeable carbons will permit higher flow rates than the lesspermeable carbons. Similarly, electrode shapes, electrode dimensions,and manner of disposal of the electrode in the electrolyte will alsohave a bearing on the flow rate. Thus, owing to the many different typesof carbon which can be employed and the almost infinite number ofcombinations of electrode shapes, dimensions, and methods of disposal ofthe electrode in the electrolyte, there are no really fixed numericallimits on the flow rates which can be used. Broadly speaking, the upperlimit on the flow rate will be that at which breakout of feedstockand/or fluorinated product begins along the immersed portion of theelectrode element. Herein unless otherwise specified, breakout isdefined as the formation of bubbles of feedstock and/or fluorinatedproduct on the outer immersed surface of the anode (electrode) withsubsequent detachment of said bubbles wherein they pass into the mainbody of the electrolyte. Broadly speaking, the lower limit of the feedrate will be determined by the requirement to supply the minimum amountof feedstock sufficient to prevent evolution of free fluorine.

While the feed introduction means is protected from electrolyte invasionby the instant invention, there must be some penetration of the pores ofat least the outermost portion of the electrode element where thereaction can take place. The above-described pressure balance willpermit some migration of electrolyte into the pores of the anode. Theamount of said migration will depend upon the inlet pressure of thefeedstock but mostly upon the pore size. The larger size pores aremainly responsible for the electrolyte invasion into the electrodeelement cavities. It has been found that porous carbon anodes asdescribed herein can be successfully operated when up to 40 to 50 percent of the pores, of what is primarily the reaction zone, have beenfilled by liquid HF electrolyte so long as the electrode passages, inwhat is primarily the feed zone, can be kept clear.

The feed material and the products obtained therefrom are retained inthe cell for a period of time which is generally less than one minute.Because the residence time is comparatively short and is especiallyuniform, the production of the desired products is facilitated. Thefluorinated products and the unconverted feed are passed from the celland then are subjected to conventional separation techniques such asfractionation, solvent extraction, adsorption, and the like, forseparation of unconverted feed and reaction products. Unconverted orinsufficiently converted feed materials can be recycled to the cell forthe production of more highly fluorinated products, if desired.Perfluorinated products, or other products which have been too highlyfluorinated, can be burned to recover hydrogen fluoride which can bereturned to the cell, if desired. By-product hydrogen, produced at thecathode, can be burned to provide heat energy or can be utilized inhydrogen-consuming processes such as hydrogenation, etc.

Referring now to the drawings, particularly FIG. 1, there is shown inschematic representation a complete electrochemical conversion cellhaving a porous electrode element having the general shape of arectangular block. A first passageway 12 extends longitudinally into andsubstantially across said block adjacent the lower end thereof. Theinterior wall of said passageway 12 comprises a first surface for theintroduction of a feed material from first conduit 14 into the pores ofsaid porous element 10. Depending upon the size and configuration ofporous element 10, more than one passageway 12 can be provided. Also, ifdesired, the feedstock can be introduced into the center of passageway12 by means of conduit 16. Said porous element 10 is disposed in a cellcontainer 18. The upper end of said porous electrode element is abovethe level of the electrolyte in said container as depicted by referencecharacter 20. Thus, the upper end surface of porous electrode element 10comprises a second surface for withdrawing unreacted feedstock andproduct from the pores of the porous electrode element 10. Conduit 22comprises a second conduit means for withdrawing product and unreactedfeedstock from within the pores of porous electrode element 10. Ifdesired, the space above the electrolyte can be divided by a partition24 extending from the top of the cell to below the level of theelectrolyte to keep the anode products separated from the cathodeproducts; or, a conventional cell divider can be employed to divide thecell into an anode compartment and a cathode compartment. However, sucha divider is not essential and can be eliminated. A current collector 26comprising a pair of metal bars extends into the top portion of porouselectrode element 10. If desired, a metal insert 28 can be provided inthe top portion of electrode element 10 to increase current collectionefficiency. A cathode 30, fabricated of any suitable metallic materialsuch as a screen, perforated plate, etc., is disposed in said cell asindicated. Cathode products can be removed via conduit 27.

In FIG. 2, porous electrode element 32 has the general shape of arectangular block. A first passageway 34 extends longitudinally into andsubstantially into and across said block adjacent to the lower endthereof. The surface of said passageway 34 comprises a first surface forintroduction of the reactive feedstock into the pores of porouselectrode element 32. First conduit 36 extends into said block and intocommunication with said first passageway 34 at about the midpointthereof. If desired, the feed conduit can be connected to one end ofpassageway 34 as indicated. However, it is preferred to plug the openend of passageway 34 and introduce the feedstock at about the midpointand said passageway by conduit 36. A second passageway 38 extendslongitudinally into and substantially across said block adjacent theupper end thereof. Said second passageway comprises a collection lateraland the surface thereof provides a second surface for withdrawingproducts and unreacted feedstock from within the pores of said porouselectrode element 32. Anode effluent conduit 40 is connected into aboutthe midpoint of passageway 38 as shown. If desired, depending upon thesize and configuration of porous electrode element 32, more than onepassageway 34 and more than one passageway 38 can be provided. Currentcollectors 42, comprising metal rods, extend into the upper end ofporous electrode element 32. Referring now particularly to FIG. 3 thereis shown in cross-sectional view the electrode element of FIG. 2. As canbe seen from FIG. 3 porous electrode element 10 is comprised of twosections, section B having large pores and section A having either smallpores or pores similar to those of section B, but which have beentreated according to the present invention.

Referring now to FIG. 4, there is shown an alternate embodiment of theelectrode assembly of the instant invention. This assembly has a porouselectrode element 44 in the general shape of a hollow tube, closed atone end thereof and open at the other end. The bottom of said porouselectrode element is sealed with a suitable resistant cement material 46such as Fluoroseal. Said porous electrode element 44 is mounted onto thelower end of a generally tubular cap 48 by means of the threads shown.Any other suitable means for attaching porous element 44 to cap 48 canbe employed. Said cap 48 can be fabricated from any suitable metal suchas brass or suitable plastic such as Teflon (trademark). A first conduit50 extends through said cap 48 into the lower portion of the interior ofporous element 44 to a point adjacent said closed end thereof. Theoutlet of said conduit 50 is thus in communication with the bottominterior surface which comprises a first surface of said porous element44 for the introduction of feedstock into the pores of said porouselectrode element 44. An annular space 52 surrounds said first conduit50 where it passes through said cap 48. A second conduit 54 extendsoutwardly from said cap 48 and is in communication with said annularspace 52. It will be noted that via said annular space 52 said conduit54 is also in communication with the upper portion of the interior wallof said porous element 44. Said upper portion of said inner wallcomprises a second surface of said porous electrode element 44 for thewithdrawal of products and any unreacted feedstock from said porouselement. The electrode assembly is shown as being positioned in a bodyof electrolyte. If desired, the region where the said porous element 44joins cap 48 can be covered with an external seal 56, such as Teflon(trademark) tape. This seal is preferably provided at the area of theelectrolyte level as shown. A metal plug 58 is mounted on the lower endportion of said first conduit 50 in a close fitting relationship withthe inner wall of said porous electrode element 44. Said metal plug thusdivides the inner wall of said porous electrode element 44 into a lowerfirst surface and an upper second surface. Said lower first surface canbe defined as comprising the chamber 60 which is formed at the lower endof conduit 50. Said upper second surface can be defined as the portion62 of the inner wall of porous element 44 which is in communication withannular space 52. This provides a positive arrangement for forcing feedmaterial into the pores of porous element 44. Said feed material is thusforced to enter the pores by said first surface (the walls of chamber60), pass upwardly through the network of pores in porous electrodeelement 44, and exit from said pores through said second surface 62. Ascan be seen from FIG. 4 the size of the pores is relatively small in thearea of lower chamber 60 and gradually becomes larger going from theportion of said electrode elements surrounding chamber 60 to the upperportion of said electrode element as depicted by upper wall portion 62which forms said second surface.

Referring now to FIG. 5 there is shown a cylindrical electrode element64 having a current collector 66 embedded in the upper, large porediameter portion thereof. Feed is introduced into cavity 68 in thelower, small pore diameter portion of electrode element 64 via line 70.Cavity 68 is sealed from the bulk of the electrolyte by means of Teflonplug 72.

Many conventional parts such as temperature controllers, electricalapparatus, recovery equipment, and the like have not been shown for thepurpose of simplicity, but the inclusion of such equipment is understoodby those skilled in the art and is within the scope of the invention.

EXAMPLE I A cylindrical porous carbon anode l-% inches in diameter and14 inches long was connected to a copper current collector at its upperend and was drilled and tapped at its lower end to provide for a fittingand connection for the invention of liquid ethylene dichloride feed.After the bottom fitting had been installed, a small void, A X inch, waspresent, located about one inch from the bottom of theanode. The anodematerial was aconventional porous carbon material (National Carbon Co.PC 45) which had an effective porosity of about 48 per cent, an averagepore diameter of about 58 microns, and a permeability of about 20darcys.

A solution of high-density polyethylene was prepared by adding as muchof the polymer as could be dissolved in boiling toluene. The anode waspreheated and the bottom l-V4 inches was impregnated by being dippedinto the polymer solution. The anode was then dried in an oven at 1 C.for one hour, and it was found that 0.5 g of polymer had been depositedin the pores of the carbon. This was about 0.03 g polymer per cc voidspace.

The treated anode was then installed, at an immersion depth of 12inches, into a stainless steel electrolytic cell which contained astainless steel cathode. The electrolyte was about 50 pounds of KF-ZHF(approximately 40 weight per cent HF and saturated with about 100 g ofUP).

The above-described cell containing the treated anode was used,essentially continuously, for 27 days for the electrochemicalfluorination of ethylene dichloride to fluorinated products includingdichlorotetrafluoroethane which is a valuable precursor to thepolymerizable tetrafluorethylene. The conditions of the conversion werean electrolyte temperature of about 96 C., an ethylene dichloride feedrate of 55-60 ml (liquid)/hour, 66.5 volts, and a current density ofma/cm. The HF was replaced as it was consumed. During this time theanode pressure did not exceed 3.5 psi.

For a control an electrode which was identical except for not beingimpregnated with polymer was immersed to a depth of 12 inches in anidentical electrochemical conversion apparatus. Within less than one daythe anode pressure exceeded 10 psi, the anode eventually plugging offthus hindering introduction of feed material. The product producedcontained excessive quantities of overly fluorinated and crackedproducts. The anode pressure is the pressure required to pass the fluidfeed through the anode in the abovedescribed system at the specifiedrate.

EXAMPLE I] Another porous anode essentially identical to that of ExampleI was treated by dipping the bottom l-r inches into a commercialsuspension of Teflon in water. The suspension contained about 60 weightper cent Teflon particles and about 0.05 to- 0.5 micron in size. Thissuspension was stabilized by the presence of a small amount of acommercial nonionic emulsifying agent (Triton X-100) of the alkylarylpolyether alcohol type. After dipping the anode was dried at 210 F. forabout l6 hours and then heated at 550 F. for one hour to destroy theemulsifying agent.

The treated anode was installed in a cell essentially identical to thatof Example I where it remained for 24 hours, being used for theelectrochemical fluorination of ethylene dichloride until operation washalted due to an unrelated malfunction. The anode was disassembled, andthe feed tube, the feed cavity, and the feed fittings were completelyfree of electrolyte. The bottom l% inches of the anode was a bluish grayand appeared not to have been wetted by the electrolyte. (Although onlythe bottom l-k inches of the carbon was dipped, some wicking action hasoccurred extending the region of treatment an additional inch.) Thistest showed that the Teflon treatment was successful in preventinginvasion of the electrode cavity.

EXAMPLE III The run of Example [I was essentially repeated except thatthe cylindrical anode was treated with an aqueous suspension ofpolyethylene.

The bottom l-k inches of the anode was impregnated with a commercialsuspension of polyethylene (Moropol 700, Moretex Chemical Products, a 30percent solids suspension of low molecular weight polyethylene in water,stabilized with a nonionic polyoxyethylene derivative of a long chaincompound). The impregnation was carried out at room temperature and 5.43g of solids was deposited in the carbon. This corresponds to about 0.3g/cc void space.

Using the same cell and general operating conditions as described inprevious examples, the treated anode was tested for the fluorination ofethylene dichloride.

The anode began operating at an anode pressure of 1-r psi. After 8 daysof continuous operation the anode pressure was still I-% psi. At thistime the cell was deliberately shut down for about 16 hours to simulatea shutdown from an equipment malfunction. With other untreated anodes,such a shutdown is generally accompanied by an increase in anodepressure when the cell is started up again. However, the treated anodewas started up and 2 days later was still operating at 1% psi.

This test showed that treatment of the anode with a suspension ofpolyethylene is also suitable.

In another similar test, the entire anode was treated with theabove-described polyethylene suspension instead of only the bottom 1%inches.

When this anode was used in the fluorination cell, it was found tooperate erratically with flashing in the cell and with the periodicproduction of product which was black in color, apparently due tocarbonization. The anode also became polarized three times in about thefirst 3 hours of operation.

The test wherein the entire anode was treated was again repeated butwith a treating suspension containing only weight per cent solids todeposit only one half as much solids in the porous carbon. Using thisanode resulted in somewhat more satisfactory operation, but there wasstill dark product formation and flashing in the cell.

These latter two tests indicate that impregnation of the entireelectrode element is not only unnecessary for satisfactory fluorination,but is actually undesirable.

EXAMPLE IV Another porous anode essentially identical to that of ExampleI was treated by still another embodiment of the invention.

A poly(vinylidene chloride) solution was prepared from 10.2 g of Saran(Dowex 242 L resin) in about 350 ml of methyl ethyl ketone at theincipient boiling point of the solution. The bottom linches of theabovedescribed anode was preheated to 75 C. and was impregnated byimmersing of the bottom 1% inches into the solution. The impregnatedanode was then placed in a tube furnace and the solvent was removed byheating in a nitrogen stream at about 100 C. The temperature was thenincreased to 500 C. and maintained there for about 3 hours to decomposethe polymer.

The impregnation, drying, and pyrolysis operation was repeated twicemore such that the increase in weight of the impregnated anode after thelast pyrolysis was about 2 grams.

The treated anode was then installed, at an immersion depth of 12inches, into a stainless steel electrolytic cell which contained astainless steel cathode. The electrolyte was about 50 pounds of KF'2HF(approximately 40 weight per cent HF and saturated with about 100 gofLiF).

The above-described cell containing the treated porous carbon anode wasused continuously for electrochemical fluorination of both methane(about l week) and ethylene dichloride (about 10 days). The conditionsof the conversion were an electrolyte temperature of about C., currentdensity of about ma/cm, voltage at about 7-9 volts, and at feed rates ofabout 1.5 moles per hour. The HF was replaced as it was consumed. Duringthis 17-day period, the anode pressure did not rise above 5 psi.

As noted by the control runs for Example I anodes of this type could notpreviously be used for such a long period of time and at such animmersion depth (12 inches) without the anode pressure rapidly exceeding10 psi or rapidly plugging ofi the flow of feed material, or producingexcessively large quantities of overly fluorinated or cracked products.

EXAMPLE V The following tests were conducted to simulate the resistanceto electrolyte invasion of a porous carbon electrode assembly having apore size at the lower region which is lower than the porosity in theupper region. A section of a relatively tight porous carbon (Stackpole139 having about 30 per cent voids, an average pore size of about 10microns, and a permeability of about 0.06 darcys) was immersed to adepth of 12 inches in a typical electro-chemical fluorinationelectrolyte and under essentially operating conditions. The cavity inthe section of porous carbon, a %-inch hole extending vertically in thecarbon section, was found to be completely free of electrolyte after a24- hour test.

Specifically, the small pore size carbon anode was a section of l X l X6% inch material into which had been drilled a Va-inch hole to withinr-inch of its end. A Teflon-covered length of copper tubing was fittedinto the open end of the hole. It was in electrical contact with thecarbon and acted as the current collector. The electrolyte was KF'2HFmaintained at about 93 C. The bottom of the anode was at a depth of 12inches from the surface of the electrolyte. For a 24-hour period, avoltage of about 5.5 volts was applied to the anode and no feed materialwas passed through the anode. The voltage was insufficient to evolvefree fluorine but was, nevertheless, considered to be satisfactory forthe validity of the electrolyte migration test.

For purposes of comparison, the same test was carried out with a porouscarbon anode section which had much larger pores (National Carbon 45having 50 per cent voids and an average pore size of about 55 microns).An anode section was made which was about 4 inches long and had a squarecross section with about I-% inches on each side. A similar 'A-inch holewas drilled to within 1% inch of the end of this anode and the hole wassimilarly fitted with a copper current collector. This anode was alsomaintained in the electrolyte under conditions essentially identical tothose described above for 24 hours. At the completion of the test, bothanodes were removed and sawed in half to determine the extent ofelectrolyte penetration.

The results of the examination showed that the fine pore carbon specimencontained no electrolyte in its cavity and there was little or noelectrolyte present in any portion of the anode. This was determined byspecific gravity determination on selected portions of the carbon. Thelarge pore carbon, on the other hand, was found to contain a largequantity of electrolyte in its cavity and density measurements showedthe presence of considerable invasion of the porous material, the extentof invasion increasing with the depth of submersion.

The above tests demonstrate the advantages of using a dual porosityporous carbon anode for electrochemical fluorination. The lower portionof the porous carbon anode requires a small pore size to resist invasionof the electrolyte and to protect the functioning of the electrodecavities. The upper portion requires somewhat greater pore size topermit some migration of electrolyte and an adequate flow of feedmaterial.

While this invention has been described in detail for the purpose ofillustration, it is not to be construed as limited thereby but isintended to cover all changes and modifications within the spirit andscope thereof.

We claim:

1. An electrode assembly, adapted to be at least partially immersed in aliquid electrolyte, comprising:

a dual porosity porous electrode element comprising a porous carbonwhich is not wetted by said electrolyte having a lower section and anupper section, said lower section having a smaller effective porositythan said upper section; and

a feed introduction means formed within said lower section andcomprising a first surface for introducing a reactive feedstock into thepores of said lower section;

and wherein the pore size in said lower section is within the range offrom 0.01 to 35 microns average diameter, with no significant amount ofpores greater than about 70 microns in diameter, and sufficiently smallto inhibit electrolyte invasion thereof and protect said feedintroduction means from electrolyte invasion, and the pore size in saidupper section is within the range of from about 40 to 150 micronsaverage diameter and sufficient to permit adequate movement of saidfeedstock therein without breaking out into the bulk of saidelectrolyte.

2. An electrode assembly according to claim 1 wherein:

the permeability of said lower section is within the range of about0.001 to 4 darcys; and the permeability of said upper section is withinthe range of about 5 to 75 darcys.

3. An electrode assembly according to claim 1 wherein the pores of saidlower section contain a small but effective electrolyte invasioninhibiting amount of a polymer.

4. An electrode assembly according to claim 3 wherein:

said lower section of said porous element comprises from 1/50 to k ofsaid element; and

said lower section contains from 0.0125 to 0.4 gram, per cubiccentimeter of void space, of a polymer selected from the groupconsisting of polymers of olefins containing from two to eight carbonatoms per molecule, and fluorocarbon polymers.

5. An electrode assembly according to claim 1 wherein the pores of saidlower section contain a small but effective electrolyte invasioninhibiting amount of pyrolyzed carbonaceous material.

6. An electrode assembly according to claim 5 wherein:

said lower section of said porous element comprises from l/SO to k ofsaid element; and

said lower section contains from 0.0125 to 0.4 gram,

per cubic centimeter of void space, of pyrolyzed carbonaceous material.

7. An electrode assembly according to claim 1 wherein:

a first conduit means is in communication with said first surface;

said upper section includes a second surface, spaced apart from saidfirst surface, for withdrawing reaction product from the pores of saidporous element; and

a second conduit means is in communication with said second surface.

8. An electrode assembly according to claim 7 wherein:

said porous element is generally rectangular in shape; and

said first surface comprises the wall of a first passageway formed insaid lower section.

9. An electrode assembly according to claim 8 wherein said secondsurface comprises the wall of a second passageway formed in said uppersection.

10. An electrode assembly according to claim 7 wherein:

said porous element is generally cylindrical in shape;

said first conduit means is in communication with,

and together with a wall of said porous element in said lower sectionforms a first chamber comprising said first surface; and

said second conduit means is in communication with the wall of a secondchamber formed in said upper section and comprising said second surface.

1 1. An electrode assembly according to claim 7 wherein:

said porous element has the general shape of a hollow tube which isclosed at one end an open at the other end;

said first conduit means extends into the lower portion of the interiorof said tube to a point adjacent said closed end to form said firstchamber comprising said first surface; and

said second conduit means is in communication with a second chambercomprising the upper portion of the inner wall of said tube.

12. A method for making a dual porosity porous electrode element,adapted to be at least partially immersed in a liquid electrolyte, saidelement having a lower section and an upper section comprising a porouscarbon which is not wetted by said electrolyte, said lower sectionhaving a smaller effective porosity than said upper section, with thepore size in said lower section being within the range of from 0.01 to35 microns average diameter, with no significant amount of pores greaterthan about microns in diameter, and sufficiently small to inhibitelectrolyte invasion thereof and protect a feed introduction meansformed therein from electrolyte invasion, and the pore size in saidupper secto 0.4 gram, per cubic centimeter of void space, of a polymerselected from the group consisting of polymers of olefins containingfrom two to eight carbon atoms per molecule, and fluorocarbon polymers.

14. A method in accordance with claim 12 wherein:

said lower section comprises from 1/50 to k of said element; and

said lower section is impregnated with from 0.0125 to 0.4 gram, percubic centimeter of void space, of pyrolyzed carbonaceous material.

1. An electrode assembly, adapted to be at least partially immersed in aliquid electrolyte, comprising: a dual porosity porous electrode elementcomprising a porous carbon which is not wetted by said electrolytehaving a lower section and an upper section, said lower section having asmaller effective porosity than said upper section; and a feedintroduction means formed within said lower section and comprising afirst surface for introducing a reactive feedstock into the pores ofsaid lower section; and wherein the pore size in said lower section iswithin the range of from 0.01 to 35 microns average diameter, with nosignificant amount of pores greater than about 70 microns in diameter,and sufficiently small to inhibit electrolyte invasion thereof andprotect said feed introduction means from electrolyte invasion, and thepore size in said upper section is within the range of from about 40 to150 microns average diameter and sufficient to permit adequate movementof said feedstock therein without breaking out into the bulk of saidelectrolyte.
 2. An electrode assembly according to claim 1 wherein: thepermeability of said lower section is within the range of about 0.001 to4 darcys; and the permeability of said upper section is within the rangeof about 5 to 75 darcys; and said porous electrode element comprisesporous carbon.
 3. An electrode assembly according to claim 1 wherein thepores of said lower section contain a small but effective electrolyteinvasion inhibiting amount of a polymer.
 4. An electrode assemblyaccording to claim 3 wherein: said lower section of said porous elementcomprises from 1/50 to 1/2 of said element; and said lower sectioncontains from 0.0125 to 0.4 gram, per cubic centimeter of void space, ofa polymer selected from the group consisting of polymers of olefinscontaining from two to eight carbon atoms per molecule, and fluorocarbonpolymers.
 5. An electrode assembly according to claim 1 wherein thepores of said lower section contain a small but effective electrolyteinvasion inhibiting amount of pyrolyzed carbonaceous material.
 6. Anelectrode assembly according to claim 5 wherein: said lower section ofsaid porous element comprises from 1/50 to 1/2 of said element; and saidlower section contains from 0.0125 to 0.4 gram, per cubic centimeter ofvoid space, of pyrolyzed carbonaceous material.
 7. An electrode assemblyaccording to claim 1 wherein: a first conduit means is in communicationwith said first surface; said upper section includes a second surface,spaced apart from said first surface, for withdrawing reaction productfrom the pores of said porous element; and a second conduit means is incommunication with said second surface.
 8. An electrode assemblyaccording to claim 7 wherein: said porous element is generallyrectangular in shape; said first surface comprises the wall of a firstpassageway formed in said lower section.
 9. An electrode assemblyaccording to claim 8 wherein said second surface comprises the wall of asecond passageway formed in said upper section.
 10. An electrodeassembly according to claim 7 wherein: said porous element is generallycylindrical in shape; said first conduit means is in communication with,and together with a wall of said porous element in said lower sectionforms a first chamber comprising said first surface; and said secondconduit means is in communication with the wall of a second chamberformed in said upper section and comprising said second surface.
 11. Anelectrode assembly according to claim 7 wherein: said porous element hasthe generAl shape of a hollow tube which is closed at one end an open atthe other end; said first conduit means extends into the lower portionof the interior of said tube to a point adjacent said closed end to formsaid first chamber comprising said first surface; and said secondconduit means is in communication with a second chamber comprising theupper portion of the inner wall of said tube.
 12. A method for making adual porosity porous electrode element, adapted to be at least partiallyimmersed in a liquid electrolyte, said element having a lower sectionand an upper section comprising a porous carbon which is not wetted bysaid electrolyte, said lower section having a smaller effective porositythan said upper section, with the pore size in said lower section beingwithin the range of from 0.01 to 35 microns average diameter, with nosignificant amount of pores greater than about 70 microns in diameter,and sufficiently small to inhibit electrolyte invasion thereof andprotect a feed introduction means formed therein from electrolyteinvasion, and the pore size in said upper section being within the rangeof from about 40 to 150 microns average diameter and sufficient topermit adequate movement of a reactive feedstock therein withoutbreaking out into the bulk of said electrolyte, which method comprises:impregnating said lower section with a small but effective amount of (1)a polymer or (2) a pyrolyzed carbonaceous material to obtain saidsmaller effective porosity in said lower section.
 13. A method inaccordance with claim 12 wherein: said lower section comprises from 1/50to 1/2 of said element; and said lower section is impregnated with from0.0125 to 0.4 gram, per cubic centimeter of void space, of a polymerselected from the group consisting of polymers of olefins containingfrom two to eight carbon atoms per molecule, and fluorocarbon polymers.